Sky News sources have also learned the militants are likely to have laid booby traps and landmines to stop the girls being found.

Video:School Where Girls Were Kidnapped

Israel has become the latest country to offer to help the search effort. Experts from Britain, France and the US are already in the country.

French President Francois Hollande has offered to host a summit in Paris this week involving Nigeria and its neighbours.

Two divisions of Nigeria's army have been sent to Borno and the country's president, Goodluck Jonathan, said this, combined with the international help, made him optimistic of finding the girls.

But there has been further criticism of the government's response to the kidnapping.

Former vice president Atiku Abubakar told Sky News: "This is a clear case of mismanagement of a small group of bandits who have been allowed to really grow into a monstrous terrorist organisation that we now have."

Boko Haram, whose name means "Western education is forbidden", has been waging an insurgency in Nigeria for the last five years.